Due to the random hate switching and the draw in attack it is highly beneficial to have the alliance gather around Hraesvelg.
This includes BLMs, WHMs, and RDMs.
Otherwise he will randomly run off to go attack someone which increases the time needed to kill him significantly.

WHMs should focus on healing everyone they can, as anyone can get hit.
2HRs were highly beneficial in our first kill of him, however keep in mind the resistance to Earth.